The present invention relates broadly to fabrication of silica-based optical devices. Background of the Invention
In optical devices it is desirable to merge silica-based optical structures with processing elements in integrated form. This can enable, for example, chips to be provided which interface between active and passive optical elements.
One problem to be overcome in the fabrication of such devices is the development of low temperature deposition processes which allow silica-based waveguides to be monolithically integrated with active signal processing elements based on materials are unable to withstand high temperatures.
Low temperature plasma-enhanced chemical vapour deposition (PECVD) techniques allow for optical processing elements to be directly incorporated on a substrate, without damaging the processing elements. However, there can be significant tradeoffs to using conventional PECVD techniques, including those adapted for low temperature operation. In conventional PECVD techniques, nitrous oxides and silane are utilised. Such elements often result in a material having a high level of absorption at a wavelength around 1.5 xcexcm. In order to remove the chemical constituents which give rise to this loss, it is necessary to heat the deposited material to higher temperatures, thereby eliminating the advantages provided by the low temperature deposition process.
At least preferred embodiments of the present invention seek to provide a modified PECVD process which seek to ameliorate the aforementioned disadvantages.
In accordance with a first aspect of the present invention, there is provided, a method of fabricating a silica-based photonic waveguide device, the method comprising the step of depositing waveguide layers containing silica using PECVD, wherein a liquid source material containing silicon is used during the PECVD deposition process and at least one layer having low optical losses in the infra-red region is produced.
The waveguide layers may be deposited at a low process temperature, preferably below 500xc2x0 C.
The PECVD may be carried out in the absence of nitrogen and nitrogen-containing source materials.
Preferably, the PECVD is carried out in an ambient comprising a mixture of oxygen and vapour from the liquid source material.
Preferably, the waveguide device is formed without high temperature thermal annealing. Where the method includes a step of annealing the device, the annealing is preferably carried out at a temperature below 800xc2x0 C.
In one embodiment, the liquid source material comprises tetraethoxysilane (TEOS).
The method may further comprise the step of controlling a refractive index contrast within the deposited waveguide layer by addition of a dopant material during the PECVD. The dopant material may be provided in a gaseous or liquid form. In one embodiment, the dopant material may comprise tetramethylgermaniun (TMG).
The method may further comprise a step of controlling at least one optical property of the waveguide layer by controlling ion-bombardment of each deposited waveguide layer during the PECVD process. The ion-bombardment may be controlled by controlling an electrode spacing during the PECVD. The ion-bombardment may further or alternatively be controlled by controlling the frequency of RF power applied to electrodes used during the PECVD process. In a preferred embodiment the step of controlling at least one optical property comprises simultaneously applying a lower frequency component of RF power and an upper frequency component of RF power to electrodes used during the PECVD, each component having a different frequency, and controlling ion bombardment by controlling the power of the lower frequency RF component so as to control the or each optical property.
The optical property which is controlled through the ion-bombardment may comprise a refractive index of the waveguide layer or stress-induced birefringence in the waveguide layer.
The method may further comprise the step of using the ion-bombardment to control stress in the deposited waveguide layer.
The method may further comprise the step of using the ion-bombardment to minimise curvature of a wafer onto which the waveguide layer is being deposited.
In accordance with a further aspect of the present invention there is provided an optical waveguide device incorporating a silica-based waveguide layer deposited using the method of the present invention.
In accordance with a further aspect of the present invention there is provided a method of fabricating a silica-based waveguide device, the method comprising the step of depositing waveguide layers containing silica using PECVD, wherein a liquid source material containing silicon is used during the PECVD.
It has been found by the applicant that in at least preferred embodiments, the present invention can facilitate the manufacture of silica-based waveguide devices at low deposition temperatures, without a requirement for using a high density PECVD process such as hollow-cathode PECVD.